(a) The Field of the Invention
The present invention relates to a process for the preparation of solutions of cellulose derivatives from which regenerated cellulose bodies may be formed by coagulation and regeneration, in particular by wet spinning. Most specifically, the derivatives in question are reaction products of cellulose and formaldehyde, and the solvents employed are organic solvents, preferably aprotic polar solvents.
Cellulose derivatives of the kind of those to which the invention relates have been prepared according to the known art and are generally considered as "methylol derivatives" and therefore this expression will be used in the present description as well. These derivatives are well identified by the presence in their infrared spectrum of an absorption band close to 945 cm.sup.-1 whose intensity increases with increasing content of formaldehyde units per anydroglucosidic unit, as titrated by the Na.sub.2 SO.sub.3 method. This absorption band could be related to C--O stretching of --O--CH.sub.2 --OH group.
There are however doubts as to the expression "methylol derivatives" accuracy from the chemical viewpoint since the chemical behaviour of the derivatives which are actually formed does not wholly correspond to that which should have been expected, according to present knowledge, from cellulose methylol derivatives, viz. from compounds having a hemiacetalic structure R--O--CH.sub.2 --OH. However said doubts are not relevant to the process according to the invention and to the results which may be achieved therefrom, and therefore the expression "methylol derivatives" will be employed to designate the derivatives to which the invention refers and should be understood regardless of any possible doubts as to its scientific accuracy.
The invention provides a process which permits the preparation of filaments and fibres or the preparation of films and other shaped bodies of regenerated cellulose, without the use of chemical agents (carbon sulfide, soda) which are characteristic of the classic viscose process.
Binary or ternary systems which are capable of dissolving cellulose and from which the cellulose itself may theoretically be regenerated are known, but such systems have a pure experimental interest or at least are not adapted to industrial applications, for various reasons. Some of them are described e.g. in Cellulose Chemistry and Technology, 9 (1975) page 265 onwards. Another known system is constituted by paraformaldehyde or formaldehyde (CH.sub.2 O) and dimethylsulfoxide (DMSO), and in this system the cellulose derivatives which form are considered, as it has been said, as methylol derivatives. In U.S. Pat. No. 4,022,631 a method for the regeneration of the cellulose from solutions in said solvent system is described and claimed, which is characterized by the use of a coagulating bath which is alkaline because of the presence of ammonium or amino compounds or alkali sulfites or thiosulfate.